This invention relates to a code division multiple access (CDMA) receiver for use in a CDMA system and a method of controlling the CDMA receiver.
As a cellular mobile radio communication system, a wide variety of multiple access systems have been heretofore proposed and be adopted in the world. Among others, a recent tendency has been directed to a cellular mobile radio code division multiple access (CDMA) system which has a specific spreading code assigned to each channel and which will be simply called a CDMA system. In such a CDMA system, a modulated wave of an identical carrier frequency which is spread by each specific spreading code is transmitted as a radio signal from a transmitter side to a receiver side. Responsive to the radio signal, a CDMA receiver in the receiver side carries out synchronization operation by the use of each specific spreading code to identify a desired channel. In order to distinguish the channels from one another, different spreading codes are used to identify radio channels between the base station and the mobile terminals.
In addition, it is to be noted that the radio signal is received through a plurality of paths, namely, a multipath in the CDMA system and, therefore, multipath fading should be removed from the radio signal in the CDMA receiver by accurately detecting a predetermined signal, such as a synchronization signal and/or a pilot signal.
Furthermore, it is to be considered in the cellular mobile radio communication system that each mobile terminal is moved through the radio service areas or cells from one to another with communication kept between each mobile terminal and a base station. In this event, the base stations must be switched from one to another without interrupting communication with each mobile terminal.
Taking the above into account, a CDMA receiver which is used in a mobile terminal has a rake receiver supplied with radio signals through the multipath and a searcher for searching such multipath signals to establish chip synchronization. In other words, the searcher is used to detect optimum reception timing from the radio signals and to inform the rake receiver of the optimum reception timing. This applies to the CDMA receiver which is used in each base station. Therefore, the following description will be mainly made about the CDMA receiver of the base station.
Practically, both the rake receiver and the searcher are given reception data signals which are subjected to high frequency amplification and frequency conversion by a high frequency amplifier and an intermediate amplifier, respectively. In this event, the rake receiver is operable in response to the reception data signals received through the multipath and comprises a plurality of finger receivers for demodulating the reception data signals by the use of spreading codes into demodulated signals. To this end, calculations are made about correlations between a spreading code of a desired channel and each received multipath signal to capture codes at each reception timing through each path. Thereafter, maximal-ratio combining is carried out to augment received signal strength. Such maximal-ratio combining is effective to alleviate an influence of the multipath fading and to improve a signal-to-noise (S/N) ratio.
On the other hand, the searcher has a plurality of correlators which are operable in response to the reception data signals and delayed spreading codes to calculate correlation values between the reception data signals and the delayed spreading codes and a plurality of adders for adding the respective correlation values to store summed up correlation values into registers. Moreover, the summed up correlation values are delivered to a reception timing judgment circuit to determine an effective path and to supply the finger circuits of the rake receiver with reception timing signals representative of the effective path or optimum reception timing.
Herein, it is to be noted that the correlators of the searcher are in one-to-one correspondence to the adders.
Recently, recent attempts have been made to widen each of the cells in such a CDMA system to reduce a frequency of handoff operations for switching the base stations from one to another. This results in enlargement of a cell radius of each cell. Such enlargement of the cell radius brings about expanding a searching range in the searcher of the CDMA receiver in each base station. In other words, the searcher must be widened in the searching range.
In order to widen the searching range in the searcher, it is necessary to increase the correlators and the corresponding adders in number. Practically, the searching range is determined by a chip number of the spreading code and must have a predetermined resolution. In consequence, the correlators and the corresponding adders should be increased in number as the searching range is widened because the number of the correlators is equal to a product of the chip number and the resolution as well as the number of the adders.
Under the circumstances, the searcher is inevitably complicated in structure as the searching range is widened, because of a large number of the correlators and the adders. In addition, it is very difficult to make each base station small in size when the correlators and the adders are increased in number.
It is an object of this invention to provide a receiver which is usable in a CDMA system, which is simple in structure and small in size.
It is another object of this invention to provide a receiver of the type described, which is capable of coping with widening a search range of a searcher without increasing correlators and adders included in the searcher.
It is still another object of this invention to provide a receiver of the type described, which is usable in a base station and which is operable at a high speed.
It is another object of this invention to provide a searcher which is capable of searching optimum reception timing from a wide searching range without an increase of correlators and/or adders.
It is yet another object of this invention to provide a method of searching optimum reception timing from a wide searching range without an increase of correlators and adders.
According to an aspect of this invention, a method is for use in searching optimum reception timing from reception data signals which are received through a plurality of paths by a searcher in a CDMA system to detect optimum reception timing. The method comprises the steps of calculating, in a time division fashion, correlation values between the reception data signals and spreading codes successively delayed, to produce summed up correlation values and determining the optimum reception timing from the summed up correlation values.
In this event, the calculation step may comprise the steps of despreading the reception data signals by using the successively delayed spreading codes to produce the correlation values and summing up each of the correlation values in the time division fashion to obtain the summed up correlation values in the time division fashion. Alternatively, the calculation step may comprise the steps of despreading the reception data signals in the time division fashion by using the successively delayed spreading codes to produce the correlation values in the time division fashion and summing up each of the correlation values to obtain the summed up correlation values in the time division fashion.
According to another aspect of this invention, a method is for use in receiving reception data signals to produce a decoded data signal in a CDMA system and comprises the steps of calculating, in a time division fashion, correlation values between the reception data signals and spreading codes successively delayed, to produce summed up correlation values, determining the optimum reception timing from the summed up correlation values, despreading the reception data signals by using the optimum reception timing to produce despread and detected data signals, carrying out maximal-ratio combining of the despread and detected data signals to produce a combined data signal, and decoding the combined data signal into the decoded data signal.
According to still another aspect of this invention, a searcher is for use in a CDMA receiver to search optimum reception timing from reception data signals which are received through a plurality of paths and comprises calculation means for calculating, in a time division fashion, correlation values between the reception data signals and spreading codes successively delayed, to produce summed up correlation values and decision means for determining the optimum reception timing from the summed up correlation values.
According to another aspect of this invention, a CDMA receiver is for use in a CDMA system to produce a decoded data signal in response to reception data signals and comprises calculation means for calculating, in a time division fashion, correlation values between the reception data signals and spreading codes successively delayed, to produce summed up correlation values, determining means for determining the optimum reception timing from the summed up correlation values, despreading means for despreading the reception data signals by using the optimum reception timing to produce despread and detected data signals, means for carrying out maximal-ratio combining of the despread and detected data signals to produce a combined data signal, and decoding the combined data signal into the decoded data signal.